Few of us have cause to question whether or not calendars are correct. We hang them on the wall to remind us of what day and month it is, or when holidays fall. In the Middle Ages, one of the most important functions of the calendar was to set the dates for important religious festivals, such as Easter. By the thirteenth century, astronomers began to notice that the calendar currently in use did not correspond to observations; most significantly, the equinoxes did not fall on the days on which they were supposed to fall. In 1582, after centuries of calls for calendar reform, the Gregorian reform was finally established.
Calendars are human constructs that allow us to track the passing of time. Certain aspects of the calendar have an astronomical basis. A day is the time it takes for the Earth to rotate one time, a month is related to the time it takes for the Moon to complete its cycle of phases, and a year is the time is takes for the Earth to revolve one time around the Sun. The difficulty with using these particular events to mark the passing of time is that they are not commensurate; that is, they do not divide evenly one into the other. For example, there are 365.2422 days within a year. If we counted only 365 days within a single year, we would soon find that the seasons were occurring at the wrong time of the year.
The problem of commensurability was recognized in antiquity. The Julian calendar, named for Julius Caesar, was established in the year 46 B.C., and was used subsequently throughout Europe. This calendar corrected the errors that had accumulated, used 365 days per year, and added one extra day every four years (the leap year). Under this scheme, the year was assumed to have 365.25 days. Although this is very close to the true number of 365.2422, even this small difference would amount to an appreciable error over a few centuries.
This error was eventually recognized during the Middle Ages. The most recognizable problem with the calendar was that the vernal (spring) equinox, a phenomenon that could be observed by a trained astronomer, was falling a few days before the traditional date of March 21. This might have been a trivial problem had the peoples of Europe not been Christian. In the fourth century, a church council had decided that Easter would fall on the first Sunday after the first full moon after the vernal equinox. Because calendars could not be mass-produced and distributed over all of Christendom every year, church officials created tables that allowed local priests to figure out when Easter should fall on any given year. The tables, however, assumed that the equinox fell on March 21, and calculated Easter based on that assumption. Because the equinox was clearly falling some days earlier than that, it was feared that Easter was being celebrated on the "wrong" date, and thus calendar reform was needed. While a concern for astronomical accuracy might have been significant to some persons, the overwhelming concern for those who proposed reforming the calendar was a religious one: ensuring that the most important religious festival of the year was celebrated at the proper time.
Despite the importance that celebrating Easter held for European society, and despite recurring proposals from astronomers, it was nearly three centuries before the calendar was finally reformed. In the thirteenth century, when reform was first seriously pursued, the Pope had been the natural authority to take on such a task. Calendar reform was, after all, primarily a religious concern, and at this time, the Pope was acknowledged throughout Europe as the head of the Christian religion. Various problems, including wars, schisms, and the Protestant Reformation, prevented the papacy from actually accomplishing the reform but in the last quarter of the sixteenth century, Pope Gregory XIII brought together a commission of clergy, mathematicians, and astronomers to reform the calendar. That reform would bear his name: the Gregorian reform.
The basic scheme of the reform came from the work of Luigi Lilio, also commonly referred to by modern authors as Luigi Giglio or Aloisius Lilius. In 1582, when the reform was to take place, the vernal equinox fell on March 11. To bring the equinox back to the traditional date of March 21, 10 days would be removed from the calendar; the dates October 5 through October 14 were dropped from that year (Friday, October 15 came right after Thursday, October 4). In addition, there would be fewer leap years. Leap years would still occur every four years, except that years ending in double zeroes that were not divisible by 400 (i.e. 1700, 1800, 1900, 2100,etc.) would not be leap years. The more complex part of the Gregorian reform was the calculation of Easter by using "epacts." An epact is the age of the Moon on January 1. Using a table of epacts in combination with astronomical tables listing the dates for new moons, the date of Easter could be calculated perpetually.
The reform was set into effect by a papal bull--an official document of the office of the Pope--entitled Inter gravissimas. In addition, a document summarizing the reform, the Compendium of the New Plan for Restoring the Calendar, had been produced and sent to various universities to inform scholars of the changes that would take place. The reform, however, was not adopted uniformly throughout Europe. Catholic countries such as Italy, Spain, Portugal, and Poland adopted the reform immediately in 1582. Catholic regions of France, Germany, Belgium, Switzerland, and the Netherlands, adopted the reform within the next two years (dropping a different ten-day period, depending on when the local authorities decided to change their calendars). Other parts of Europe would not change their calendars until the eighteenth century, while nations in other parts of the world did not adopt the calendar until the twentieth century. One famous result of the delay in adopting the calendar was that the 1908 Imperial Russian Olympic team missed the competition because they did not arrange their travel plans in accordance with the Gregorian dates. Today, the calendar is nearly universal, though some religious calendars (such as the Jewish and Islamic) are kept concurrently.
Why was it that the reform was not universally embraced when it was originally mandated in 1582? Influenced by the Protestant Reformation, which had been underway for a number of decades, numerous Protestant rulers throughout Europe did not wish to adopt a reform dictated by the Roman Catholic Church. There was little scientific objection to the reform, as the modification of leap years accomplished the basic task of reconciling the incommensurability of the length of the day and year, while the scheme of epacts was an effective way to establish the date of Easter. Objections were most often framed in political terms stating that the Pope had no authority to enact a reform. This would remain a sticking point for a number of decades.
Some prominent scientists, such as J. Scaliger (1484-1558) and Michael Mästlin (1550-1631), did pose idiosyncratic objections to the reform, usually based on simplifications of astronomical calculations that the reform utilized. None of these objections was especially significant in the long run. The primary Catholic defender of the reform was Cristoph Clavius (1537-1612), a Jesuit mathematician and astronomer, who had been on the papal commission that had approved the reform; he argued that the simplifications were acceptable and indeed necessary for those not trained in astronomy to be able to use the calendar effectively. Two preeminent Protestant astronomers, both imperial mathematicians, also endorsed the reform: Tycho Brahe (1546-1601) immediately began to use the new calendar, while Johannes Kepler (1571-1630) argued that the Gregorian reform was the most effective one available.
In the end, it was recognized that both diplomacy and commerce could be more effectively pursued if all parties operated under the same calendar. In Germany, Gottfried W. Leibniz (1646-1716) advocated a reform based on the proposal of the astronomer Erhard Weigel. This reform, established in 1700,made the same changes as the Gregorian reform with regards to dropping days from the calendar and adjusting the leap years, but used Kepler's Rudolphine Tables, rather than the epacts, to calculate Easter (thus avoiding using a Roman Catholic method for determining the date of Easter, and thereby denying the authority of the Pope in such matters). This brought the calendar into step with countries that had adopted the Gregorian calendar, with the exception that Easter occasionally fell on different dates under the two calendars (this problem was corrected later in the century). In England, the change would not come until the middle of the eighteenth century, when the British government passed a bill in 1752. The days of September 3 to September 13 of 1752 were dropped from the calendar in England and its colonies (eleven days had to be dropped because1700 had been a leap year under the Julian calendar in England, but not under the Gregorian calendar).
Though the calendar is in large part based on astronomical phenomena, it is ultimately an artificial construct designed to meet the needs of human society. The Gregorian reform of the calendar in the sixteenth century was enacted for religious reasons, and, in some places, was rejected on religious and political grounds. Where it had been rejected on such grounds, it was eventually adopted for practical purposes, as the religious objections no longer carried the political import that they had originally.